Variable speed mechanism

ABSTRACT

A controlled differential-gear system includes an input differential, an output differential, and an idler gear positioned between the two differentials to operate the output differential as a subtractive type when the input differential operates as an additive type. Brake means is arranged to brake one or the other side of the differentials to vary the output speed through an infinite range from maximum reverse, through zero, to maximum forward. Three arrangements of the differentials and two embodiments of the brake means are illustrated in the drawings.

United States Patent Olcer 1 Feb. 29, 1972 [54] VARIABLE SPEED MECHANISM[72] Inventor: Behzat Olcer, 1810 Cumberland St.,

Rockford, Ill. 61 103 [22] Filed: Apr. 27, I970 [21] Appl. No.: 32,259

[52} US. Cl ..74/682, 74/756 [51] ..Fl6h 37/06 [58] Field ofSearch..74/682,687,756, 7 57 [56] References Cited UNITED STATES PATENTS3,242,769 3/ 1966 Johnson ..74/682 2,973,669 3/1961 Quigley.....2,924,122 2/1960 Foster ..74/682 3,079,813 3/1963 Quigley ..74/687FOREIGN PATENTS OR APPLICATIONS 517,119 2/1955 Italy ..74/682 620,7233/1949 Great Britain ..74/682 Primary Examiner-C. J. HusarAttorneyMcCanna, Morsbach, Pillote & Muir [57] ABSTRACT A controlleddifferential-gear system includes an input differential, an outputdifferential, and an idler gear positioned between the two differentialsto operate the output differential as a subtractive type when the inputdifferential operates as an additive type. Brake means is arranged tobrake one or the other side of the differentials to vary the outputspeed through an infinite range from maximum reverse, through zero, tomaximum forward. Three arrangements of the differentials and twoembodiments of the brake means are illustrated in the drawings.

7 Claims, 4 Drawing Figures Pdtsmmriazs m2 3.645.152

SHEET 1 BF 3 FIG.1.

m VE ,10 R M 01 01w hel gkntm ATTORNEYS PAIENTEnrwee I912 3.645.152

SHEET 2 BF 3 HG. Z. 129

FIG. 4.

lur ak The invention broadly pertains to machine elements andmechanisms. More particularly, the invention pertains to improvedvariable speed mechanisms of the type known as controlleddifferentialgear systems.

The prior art teaches that a differential-gear system alone will notproduce a variable speed drive, but must be supplemented by some othervariable speed mechanisms to control the relative rotation of variousgears. This is commonly done by adding to the differential gearing acontrol mechanism based upon the principle of the V-belt or chain drivewith adjustable cone pulleys. The relative capacity of a combined driveand differential gear system depends upon the ratio range of the driveand the required ratio range of the differential system. Differentialgearing in this case is fundamentally a system where two driving members(at least one of which is variable in speed) combine their respectiverotations and control the speed of the driven member. The differentialdesign may be such that the driving members are additive or subtractivein their influence on the driven member. With an additive-typedifferential both driving members add their respective powers to thedriven member as output power. With a subtractive-type differential thedriving members subtract their respective powers to the driven member asoutput power. In the latter case there is circulating power which mayexceed the output power. Circulating power exists when the sum of theinput powers exceeds the output power.

The prior art also teaches the use of double differential-gear systems.With the double-differential type, the torques transmitted through thevariable elements do not vary so widely as with the single-differentialtype. A double-differential system may use a V-belt or chain controlmechanism. Assuming equal ratios in all the gear sets of such anarrangement, equal forward to reverse speed is obtainable by changingthe variable speed transmission an amount or range either side of themean position. By selecting unequal ratios in the gear sets,substantially any desired variable speed range may be obtained. It isdesirable, however, to provide a variable speed mechanism without theuse of a'V-belt or chain control mechanism.

Another double-differential system is shown in U.S. Pat. No. 2,973,669issued Mar. 7, 1961 to B. T. Quigley. That apparatus, however, utilizesa clutch arrangement to selectively effect neutral, forward or reverseoperating ranges. It is desirable to provide a variable speed mechanismwhich can obtain neutral, forward and/or reverse operating rangeswithout the use of a clutch or similar apparatus.

SUMMARY The present invention relates to improvements in variable speedmechanisms of the type known as controlled differential-gear systems.

It is an object of the present invention to provide a variable speedmechanism which has the above-mentioned desirabilities and yet is ofsimplified structure.

Another object is to provide a mechanism of the type described whichemploys a novel combination of gear connections and control meanswhereby an output shaft may rotate in either forward or reversedirections at variable output speeds and also has a neutral positionwhere the output shaft is stationary.

Still another object is to provide a variable speed mechanism in whichthe output shaft can be rotated at an infinite number of speeds fromforward through neutral to reverse.

Yet another object of the present invention is to provide apparatus inaccordance with any of the above objects and which includes a pair ofdifferentials so arranged that one operates as an additive differentialand one as a subtractive differential.

These, and other objects and advantages of the present invention, willbecome apparent as the same becomes better understood from the followingdetailed description when taken with the accompanying drawings.

DRAWINGS FIG. 1 is a diagrammatic view of one embodiment of theinvention;

FIG. 2 is a diagrammatic view of a second embodiment of the invention;

FIG. 3 is a diagrammatic view of a third embodiment of the invention;and

FIG. 4, on sheet two of the drawings, is a view, partly sectional andpartly diagrammatic, of another braking apparatus usable in theinvention and utilizing a single pump.

DESCRIPTION As stated above, the apparatus of the present inventiongives variable speed outputs in an infinite range covering reverse andforward speeds through neutral where the output is zero. This result isobtained by the combination of two differential-gear units and a brakingmeans on at least one of the side members of the differential. In theembodiments illustrated, bevel gear differentials are used but it is tobe understood that other types of differential units can also be used.The differentials are arranged so that when the braking means is ineffect, the input differential functions as an additive type while theoutput differential functions as a subtractive type.

Reference is now made more particularly to the drawings which illustratethe best presently known mode of carrying out the invention and whereinsimilar reference characters indicate the same parts throughout theseveral views.

In the embodiment of FIG. 1, the input and output shafts are inside-by-side parallel relation. The input differential mechanism,generally designated 10, has a drive shaft or input shaft 11 rigidlyconnected with spider shafts l2 and I3. Planetary gear means in the formof spider gears 15 and 17 are rotatably mounted on the spider shafts l2and 13, respectively, and movable through a planetary path. A bevel gear19 is rigidly connected with a side gear 23 and both rotate freelyaround the input shaft 11. Similarly, a bevel gear 21 and a side gear 25are also rigidly interconnected with each other and can freely rotatearound the input shaft 11. As shown, bevel gears 19 and 21 are in meshwith the planetary gear means.

An output differential mechanism generally designated 29, has a drivenshaft or output shaft 31 rigidly connected with spider shafts 32 and 33.Spider gears 35 and 37 are rotatably mounted on the shafts 32 and 33,respectively. In an arrange ment similar to the input differential, abevel gear 39 and side gear 43 are rigidly interconnected and freelyrotate around the output shaft 31. Bevel gear 41 and side gear 45 arealso rigidly interconnected and freely rotate around the output shaft.

The above-described differential units 10 and 29 are mounted inside-by-side relation with shafts 11 and 31 parallel. Side gears 23 and43 mesh at one side while side gears 25 and 45 both mesh with an idlergear 27 rotatably mounted on shaft 28. Idler gear 27 is an importantelement of the present invention because of the totally differentresulting function which is hereafter described in detail. Because ofthis idler gear, side gear 45 of the output differential 29 rotates in adirection opposite the direction of rotation of side gear 43. This makesthe output differential a subtractive type differential as willhereafter become apparent.

A control system is combined with the above-described differentials. Inthe embodiment illustrated in FIG. 1, two hydraulic gear pumps 47 and 49are arranged to be driven by the side gears 25 and 23 of the inputdifferential. These pumps can also be driven by any gear in mesh witheither of the side gears 25 and 23, for example side gears 45 and 43 ofthe output differential. Preferably, however, the pumps are connected toopposite sides of the above-described combination. The pressure sides ofthe two pumps 47 and 49 are connected by proper piping to openings 57and 59 in a valve body 51. A plunger 53 is slidable within the body 51,as diagrammatically illustrated. Suitable piping connects the valve bodywith a reservoir 55 and the reservoir with the suction sides of thepumps 47 and 49. The design of the control valve is such that at anyposition ofthe plunger 53 only one port 57 or 59 will be closed. Whenone of the ports is completely closed or partially restricted, the otherwill remain open. In a neutral position, both ports can be completelyopen as shown in FIG. 1.

FIG. 2 shows another embodiment of the present invention in which thereis a coaxial arrangement of the input and output differentials. In thisFIGURE, parts of the differential portion of the embodiment having thesame function as that described above are given the same numerals of aseries 100 higher than the similar functioning parts in FIG. 1. Thiswill allow one skilled in the art to readily compare the functions whilecontrasting the structure. The input differential 110 has an input shaft111 rigidly connected to spider shafts 112 and 113. Spider gears 115 and117 are rotatably mounted on the respective shafts, as shown. Bevel gear121 and side gear 125 are rigidly connected and can freely rotate aroundthe input shaft 111. Bevel gear 119 of the input differential 110 isrigidly connected with a bevel gear 139 of an output differential unit129 and to a side gear 143. An output shaft 131 and the input shaft 111find bearing in this rigid combination of members of both differentialunits. The output shaft has a rigid connection to spider shafts 132 and133 on which are rotatably mounted spider gears 135 and 137,respectively. A bevel gear 141, in mesh with both gears 135 and 137, isrigidly connected with a side gear 145 and the two gears are rotatablymounted on the output shaft 131. A third shaft 151 is mounted parallelto the input and output shafts of the differential units and has gears125 and 125" keyed thereto. In the embodiment illustrated, gear 125' isin mesh with gear 125. Gears 125 and 145 are interconnected by an idlergear 127 rotatably mounted on shaft 128.

The previously described hydraulic control system is connected to thiscoaxial arrangement of the differential units in any convenient mannerto achieve the aforedescribed function. That is, braking one side of thedifferential units or the other side thereof. In the embodimentillustrated, pump 49 is conveniently driven by gear 125 while pump 47 isconveniently driven by gear 143 as through a gear 149 convenientlyrotatably mounted on shaft 151.

In considering the operation of the embodiments of FIGS. 1 and 2, wewill first assume that the speed ratios of the various gears equalunity. More particularly, in FIG. 1 the speed ratio of gear 23 to gear43 equals gear 25 to gear 45 which equals unity. Similarly, in FIG. 2the speed ratio of gear 125 to 125 is equal to gear 125" to gear 145which is equal to unity. With such arrangements, the variable speeddrives are capable of giving Outputs from zero to a forward and reversespeed equal to the input speed as a maximum. For example, if a constantinput of 1,000 r.p.m. is provided on shaft 11 or 111, output shaft 31 or131 can be rotated at any speed from 1,000 r.p.m. in reverse to 1,000r.p.m. in a forward direction using the above given gear ratios.

As examples of the intermediate speeds easily obtained, we will assumethe same constant input of 1,000 r.p.m. on input shaft 11 of FIG. 1.Assume that plunger 53 is moved to a position to restrict the freerotation of pump 49 thereby limiting the rotation of gear 23 to 200r.p.m. By the differential action, gear 25 will then be rotated at 1,800r.p.m. Gear 43 will also be rotated at 200 r.p.m. but in an oppositedirection; while gear 45 is rotated at 1,800 r.p.m. but in the samedirection as gear 25. As explained above, differential 29 thus becomes asubtractive differential and the spider arms 32 and 33 with output shaft31 will be rotated at one-half the sum of the speeds of gears 43 and 45.That is, one-half (1,800-200) or 800 r.p.m. By completely braking gear23, the output speed will be 1,000 r.p.m. Conversely, by completelybraking gear 25 the output speed will be minus 1,000 r.p.m. If gear 25is braked to 200 r.p.m., the output speed will be minus 800 r.p.m. Ifneither gear 23 nor gear 25 is braked, the output speed will be zero.

If it is desired that there be a change in maximum output speed, theaforementioned gear ratios can be changed. For example, with an inputspeed of 1,000 r.p.m. on shaft 11 and a desired maximum output speed of500 r.p.m. on shaft 31, the

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speed ratios of the gears in FIG. 1 should be as follows: gear 23 togear 43 equals gear 25 to gear 45 equals one-half. In this manner, theapparatus can be designed to accommodate any desired ratio of inputspeed to maximum forward or reverse output speed.

In FIG. 2, however, a different result is achieved. It will beremembered that spur gears 119 and 139 are connected for simultaneousrotation. It is not, therefore, possible to have a gear reductionbetween these two bevel gears. As a consequence, if the speed'ratio ofgear to gear 125 or of gear 125" to gear is made one-half, the speedrange will be minus 500 to 1,000 r.p.m. with a 1,000 r.p.m. input. Ifboth of the aforementioned ratios are made one-half, the speed rangewill be minus 250 to 1,000 r.p.m. These may be desirable speed rangesfor certain applications. However, if it is desired to utilize a coaxialarrangement of the input and output shafts and also have a speed changefrom the input speed to the maximum output speed with the forward andreverse maximums equal, the embodiment of FIG. 2 cannot be used.

FIG. 3 is an improved design of the coaxial combination which can obtaina speed range having equal forward and reverse speeds but of a differentmaximum from the input speed. The improvement in FIG. 3 is accomplishedby eliminating the rigid interconnection between the differential units.In the embodiment illustrated, similar parts to that in FIG. 1 areindicated by the same numerals of a series 200 higher.

In the embodiment illustrated in FIG. 3, two differential units 210 and229 are mounted coaxially. Input differential 210 has an input shaft 211and spider gears 215 and 217 rotatably mounted on spider arms 212 and213, respectively. Bevel gear 219 and side gear 223 are rigidlyconnected as are bevel gear 221 and side gear 225. Spider gears 235 and237 of the output differential 229 are rotatably mounted around thespider arms 232 and 233 of the output shaft 231. A bevel gear 239 isrigidly connected to side gear 243 and bevel gear 241 is rigidlyconnected to side gear 245. A shaft 251 is mounted in parallel to inputshaft 211 and output shaft 231. A gear 225' is keyed on shaft 251 and inmesh with gear 225. Another gear 225" is also keyed to shaft 251 and isin mesh with an idler gear 227 which also meshes with gear 245. Gears249 and 257, in mesh with gears 223 and 243 respectively, are rigidlymounted on a sleeve 259 which is rotatably mounted on shaft 251. Theaforedescribed control means can be connected in any desired manner and,in the embodiment shown, is operatively connected to gear 225' and gear249. With this arrangement, the operative characteristics in FIG. 1 isduplicated with the shafts in a coaxial arrangement.

A more complete description of the operation of the aforedescribedembodiments will now be given. Assume the plunger 53 of the controlvalve is set in the position shown in FIG. 1 with both ports 57 and 59completely open. The variable speed drive will then have an output ofzero. For example, when the input shaft 11 is rotated with a constantr.p.m. as by an electric motor M, both side gears 23 and 25 will berotated in the same direction and with the same speed as the input. Inthis case the input differential unit will act as a simple pinion andnone of its rotating members will move relative to the others. Therotation of side gear 23 will be transmitted to side gear 43 and therotation of side gear 25 will be transmitted to side gear 45 via idlergear 27. Assuming equal gear ratios, the rotation of the side gears 43and 45 will be equal but opposite. This means bevel gears 39 and 41 willrotate with equal but opposite speeds. Spider gears 35 and 37 will alsorotate around the spider shafts without any rolling action on any of thebevel gears 39 and 41. The output at shaft 31 will then be zero. Whenthe ports 57 and 59 are completely open, the hydraulic fluid in thesystem will be freely circulated without any build up of pressure. Inother words, no braking effect will be applied to the side gears 23 or25.

When the plunger 53 is moved to the right and port 5) is closed, thecirculation of pump 49 will be stopped and side gear 23 will be heldstationary along with bevel gear I). l'his will cause a doubling of thespeed of bevel gear 21 and side gear 25. This speed will be transmittedto side gear 45 and to bevel gear 41. The spider gears 35 and 37 willstart to roll on bevel gear 39 which is also held stationary by means ofthe rigid connection with side gear 43 which is in mesh with the brakedside gear 23. The rolling spider gears 35 and 37 will carry the spidershafts and impart rotation to output shaft 31. The speed of the outputshaft will be equal to half the speed of bevel gear 41 and the directionof rotation will be in the same direction as the input.

When the port 59 is partially restricted by the plunger 53, pressurewill buildup between the outlet of the pump 49 and the port 59 creatinga braking effect on the side gears 23 and 43 and decreasing theirspeeds. One example of this is given above. By various settings of thecontrol plunger 53, various braking effects can be accomplished and aninfinite number of output speeds achieved. When port 59 is restricted,the output speed is always in the same direction as the input speed.

When the above-described control is applied to the port 57, the brakingeffect will be applied on side gear 25 thereby reversing the directionof the output shaft 31.

The above-described embodiments of the variable speed drive will reactto the control system and a smooth and nearly instantaneous changebetween reverse and forward is possible. It is to be understood that thespeed of changing the direction of rotation will depend on the inertiaof the load applied and other variable factors.

When the speed ratios of the gears are changed, the output torque willbe increased or decreased proportionately as the output speed rangedecreases or increases, respectively.

In a manner similar to that described for the embodiment of FIG. I, theembodiments shown in FIGS. 2 and 3 are operatively identical. All of theembodiments are effective by using a schematically shown hydrauliccontrol system having two gear pumps, one of which is always ineffectiveinsofar as the braking action of the other is concerned. It is possible,therefore, to have a single pump and one hydraulic circuit arranged witha clutch mechanism to connect it to one side or the other. In such acase, the hydraulic circuit will have a simpler flow restricting valve.

It will be understood that the load applied on the output shaft 31, 131or 231 must be overcome to be able to obtain a rotation. This load canbe variable or constant in nature. In any case, as long as it remainswithin the capacity of the variable speed mechanism, depending on themagnitude of the load, the output motion will occur whenever thealgebraic sum of the powers of the side members of the outputdifferential equals the power required to drive the load. Since thisabovementioned algebraic sum is a function of the input power and thebraking power, it will always be determined by the load applied, and incase of variable loads the control valve settings will vary accordinglyto obtain the same rpm. at different load conditions. Once the motionstarts at a certain valve setting the remaining valve stroke will stillcover the infinitely variable speed range in both reverse and forwarddirection, as long as the braking and driving means are selectedaccording to the design capacity. Therefore, the subject variable speedmechanism is load responsive. This fact gives the mechanismpossibilities for use with variable input speeds (such as engines). Insuch a case the variable speed mechanism can be used as an automatictransmission.

FIG. 4 illustrates another control unit combined with a synchromesh gearclutch. As shown, shaft 72 is positioned parallel with input shaft 11, 111 or 211. Gears 60 and 62, connected with gears 64 and 66,respectively, are rotatably mounted at opposite ends of the shaft 72. Ifutilized in FIG. 1, gear 60 would be in mesh with gear 25 and gear 62 inmesh with gear 23. Internal gears 68 and 70 are interconnected and keyedor splined on the shaft 72, but arranged for sliding movement so thatthey can be selectively engaged with gear 64 and 66. Two friction discs74 and 76 are also keyed or splined on shaft 72 and also arranged forsliding movement along the shaft. Ball'type detents 78 and 80 aremounted on discs 74 and 76, respectively. When the common body of gears68 and 70 is moved to the right, detent is engaged and moves the discs76 to the right. They will move together until the friction surface 77contacts the friction surface 67. When this contact occurs, the rotationof gear 66 is transmitted to shaft 72 through discs 76. Gear 70 rotatesbecause of its blind connection to the shaft. When gear 66 and gear 70have obtained the same r.p.m. a further movement of gear 70 willovercome the spring-loading of the detent 80 and gears 70 and 66 willbecome meshed. The detent 80 can snap into a matching groove 81 and holdthe gears 66 and 70 in meshed position. When the sliding movement is tothe left, gears 64 and 68 can be meshed.

When rotated, shaft 72 will rotate a hydraulic pump 48 and pump fluidfrom a reservoir 56 through suitable conduits to a control valve 58.Variable speed outputs can be obtained by different setting of the valve58.

When a reverse rotation of the output shaft is required, the left-handside gears of the differentials must be braked. Preferably the gears 60and 62 are in mesh with gears of the input differential so that, in bothpositions, shaft 72 will rotate in the same direction.

Any of the aforementioned pumps 47-49 can be replaced by a mechanicalbrake, a magnetic particle brake, or any other suitable apparatus toachieve an adjustable braking effect whenever required.

With this invention the additive-type input differential adds thebraking power to the input power and gives it as an output power at itsdriven side member. The meshing side gears transmit this power to thesubtractive-type output differential. The output differential is asubtractivetype by means of the idler gear which causes the side membersof the output differential to rotate in opposite directions, while theside gears of the input differential rotate in the same direction.

While several embodiments of the invention have herein been illustratedand described, this has been done by way of illustration and notlimitation, and the invention should not be limited except as requiredby the scope of the appended claims.

I claim:

1. In a variable speed mechanism including a drive shaft; a drivenshaft; and an input differential mechanism including planetary gearmeans arranged to be driven through its planetary path by the driveshaft, and first and second gears in mesh with the planetary gear means;the improvement comprising: an output differential mechanism includingsecond planetary gear means arranged to drive the driven shaft as itmoves through its planetary path, and third and fourth gears in meshwith the second planetary gear means; first means operatively connectingthe first and third gears for rotation in opposite directions; secondmeans operatively connecting the second and fourth gears for rotation inthe same direction; whereby the input differential is an additivedifferential and the output differential is a subtractive differential;and brake means selectively operative on both of the first and secondmeans to retard the rotation thereof and vary the speed of the driveshaft from maximum reverse speed through zero output to maximum forwardspeed, the brake means comprising a synchromesh gear clutch apparatushaving first and second gears each in mesh with one of the first andsecond means and so constructed and arranged that its gears alwaysrotate in the same direction, and the synchromesh gear clutch apparatusincluding a slidable gear selectively engageable with the synchromeshgear clutch apparatus first and second gears, and means forsynchronizing the rotation of the slidable gear with the first andsecond gears prior to said engagement.

2. A variable speed mechanism as set forth in claim I wherein thedifferential mechanisms are arranged in side-byside relation, the firstmeans includes two gears directly in mesh so that the first and thirdgears rotate in opposite directions, and the second means includes twogears and an intermediate idler gear so that the second and fourth gearsrotate in the same direction.

3. A variable speed mechanism as set forth in claim 1 wherein the driveshaft and driven shaft are coaxially arranged; the differentialmechanisms are arranged in tandem; and at least one of the first andsecond means includes a third shaft disposed parallel to the drive anddriven shafts, and gears carried on the third shaft for rotationtherewith.

4. A variable speed mechanism as set forth in claim 3 wherein the otherof the first and second means includes a fourth shaft disposed parallelto the drive and driven shafts, and gears carried on the fourth shaftfor rotation therewith.

5. A variable speed mechanism including an input shaft; an output shaftcoaxial with the input shaft; an input differential mechanism includingat least one planetary gear driven through its planetary path by theinput shaft, and first and second gear means in mesh with the planetarygear; an output differential mechanism including at least one otherplanetary gear for driving the output shaft as it moves through itsplanetary path, and third and fourth gear means in mesh with the otherplanetary gear; said differential mechanisms being arranged in tandem;first interconnecting means for interconnecting said first and thirdgear means for rotation in opposite directions; second interconnectingmeans for interconnecting said second and fourth gear means for rotationin the same direction; one of the first and second interconnecting meansincluding a third shaft disposed parallel to the input and output shaftsand gears carried on the third shaft for rotation therewith; one of thegear means including an idler gear to achieve the rotational difference;whereby the input differential is an additive differential and theoutput differential is a subtractive differential; and selectivelyoperable means for braking at least one of the gear means to retard therotation thereof and vary the speed of the driven shaft.

6. An apparatus according to claim 5 wherein the differential mechanismsare bevel gear differentials, the first and second gear means eachinclude a bevel gear and a spur gear both rotatably mounted on the inputshaft and both interconnected for simultaneous rotation, and the thirdand fourth gear means each include a bevel gear and a spur gear bothrotatably mounted on the output shaft and interconnected forsimultaneous rotation.

7. An apparatus according to claim 6 wherein the spur gears of the firstand third gear means are meshed, and wherein the idler gear is meshedwith the spur gears of the second and fourth gear means.

llllllill "UNI

1. In a variable speed mechanism including a drive shaft; a drivenshaft; and an input differential mechanism including planetary gearmeans arranged to be driven through its planetary path by the driveshaft, and first and second gears in mesh with the planetary gear means;the improvement comprising: an output differential mechanism includingsecond planetary gear means arranged to drive the driven shaft as itmoves through its planetary path, and third and fourth gears in meshwith the second planetary gear means; first means operatively connectingthe first and third gears for rotation in opposite directions; secondmeans operatively connecting the second and fourth gears for rotation inthe same direction; whereby the input differential is an additivedifferential and the output differential is a subtractive differential;and brake means selectively operative on both of the first and secondmeans to retard the rotation thereof and vary the speed of the driveshaft from maximum reverse speed through zero output to maximum forwardspeed, the brake means comprising a synchromesh gear clutch apparatushaving first and second gears each in mesh with one of the first andsecond means and so constructed and arranged that its gears alwaysrotate in the same direction, and the synchromesh gear clutch apparatusincluding a slidable gear selectively engageable with the synchromeshgear clutch apparatus first and second gears, and means forsynchronizing the rotation of the slidable gear with the first andsecond gears prior to said engagement.
 2. A variable speed mechanism asset forth in claim 1 wherein the differential mechanisms are arranged inside-by-side relation, the first means includes two gears directly inmesh so that the first and third gears rotate in opposite directions,and the second means includes two gears and an intermediate idler gearso that the second and fourth gears rotate in the same direction.
 3. Avariable speed mechanism as set forth in claim 1 wherein the drive shaftand driven shaft are coaxially arranged; the differential mechanisms arearranged in tandem; and at least one of the first and second meansincludes a third shaft disposed parallel to the drive and driven shafts,and gears carried on the third shaft for rotation therewith.
 4. Avariable speed mechanism as set forth in claim 3 wherein the other ofthe first and second means includes a fourth shaft disposed parallel tothe drive and driven shafts, and gears carried on the fourth shaft forrotation therewith.
 5. A variable speed mechanism including an inputshaft; an output shaft coaxial with the input shaft; an inputdifferential mechanism including at least one planetary gear driventhrough its planetary path by the input shaft, and first and second gearmeans in mesh with the planetary gear; an output differential mechanismincluding at least one other planetary gear for driving the output shaftas it moves through its planetary path, and third and fourth gear meansin mesh with the other planetary gear; said differential mechanismsbeing arranged in tandem; first interconnecting means forinterconnecting said first and third gear means for rotation in oppositedirections; second interconnecting means for interconnecting said secondand fourth gear means for rotation in the same direction; one of thefirst and second interconnecting means including a third shaft disposedparallel to the inPut and output shafts and gears carried on the thirdshaft for rotation therewith; one of the gear means including an idlergear to achieve the rotational difference; whereby the inputdifferential is an additive differential and the output differential isa subtractive differential; and selectively operable means for brakingat least one of the gear means to retard the rotation thereof and varythe speed of the driven shaft.
 6. An apparatus according to claim 5wherein the differential mechanisms are bevel gear differentials, thefirst and second gear means each include a bevel gear and a spur gearboth rotatably mounted on the input shaft and both interconnected forsimultaneous rotation, and the third and fourth gear means each includea bevel gear and a spur gear both rotatably mounted on the output shaftand interconnected for simultaneous rotation.
 7. An apparatus accordingto claim 6 wherein the spur gears of the first and third gear means aremeshed, and wherein the idler gear is meshed with the spur gears of thesecond and fourth gear means.